Boomstick Desperado Scum
 Hive Scum Build - Warhammer 40k: Darktide 

The general idea here is to leverage the Both Barrels blessing on the double-barrel shotgun to get very fast reload speed for more unlimited-ammo blasting during Desperado. Killing close-range targets gives 30% reload speed from Speedloader in addition to the 70% from Both Barrels to make you reload twice as fast as normal. Ammo Jack rounds up, so you get 3 rounds per reload. From my testing, it seems that Both Barrels will only proc if you have 2+ rounds in the magazine when you shoot; it doesn't work if you get the kill with your 3rd shot, even while aiming. If you prefer hipfire over ADS, you can take Speedload blessing instead of Both Barrels.

Second shotgun blessing can be your choice of Deathspitter, Man-Stopper, or Flechette. The "strength" bonus of Deathspitter scales damage, stagger, and cleave. Man-Stopper gives practically unlimited cleave on crits. Flechette gives 6 bleed stacks (equivalent to a Veteran's shredder grenade) to all targets on crit. The build can maintain up to 60% crit chance for the shotgun, which should be good enough for proccing the latter two blessings.

Melee weapon is down to personal preference. I chose the shivs for mobility, attack speed, crit scaling, and effectiveness against crushers, but I think most weapons would work fine. You definitely want something that's decent against crushers because the shotgun is ineffective against them.

I picked rending on the cartel stimm for better damage against flak; that will bring you to 30% when paired with Pick Your Targets on Desperado. The shotgun has poor crit/weakspot damage scaling, so I don't think it's worth picking the finesse or crit nodes on the stimm unless you're trying to buff your melee DPS.

Gunslinger aura isn't necessary since you have unlimited ammo during Desperado and can also restore your ammo reserves to 20% with Pickpocket. In Havoc, you probably want to bring Gunslinger for your teammates anyway. In Auric and below, I would just stick with the Anarchist aura for more better melee DPS and possibly blessing procs (depending on how you built your shotgun).

I picked Sticky Hands for reduced spread on the shotgun to kill specialists and elites more reliably. Unload isn't worth taking on this build because the "magazine spent" buff resets on each reload and our first shot while aiming will use two-thirds of the magazine, so you'd just get a partial bonus on the third shot.

Nimble can be swapped for Slippery Customer if you don't like the super-long dodges.

Maxed Out Chems is a flex point. It's probably optimal, but there are other good options if you don't want to watch your stimm cooldown like a hawk. Alternatives: A Tertium Welcome, Calling for a Time Out, Long Lasting, Swift Endurance, Chem Fortified. Hyper-Critical is also good, but I would swap Precognition for Riposte on the shivs if you go that route.